LOS ANGELES — NASA's Opportunity rover ground into a rock outcrop on Mars, and excited scientists prepared to examine its makeup to learn whether it was formed under watery conditions favorable to life, mission members said Tuesday.

The robotic field geologist used its rock abrasion tool to grind a sixth of an inch (4 millimeters) into the surface of a rock dubbed "El Capitan," project manager Richard Cook said.

The rover then began to inspect the round hole with other instruments on its robotic arm. Those instruments are designed to reveal the mineralogical and elemental composition of the freshly exposed rock, as well as photograph it in microscopic detail.

'Christmas Eve' on Mars
"It's Christmas Eve, and all the scientists are anxious and excited with anticipation for what they are going to show," deputy main scientist Ray Arvidson said of the measurements.

Results were expected to take several days to reach Earth.

The rock outcrop is about the height of a curb and rings a portion of the crater that cradles the unmanned rover. Previous microscopic images show fine layering in the rock, including "El Capitan," as well as mysterious BB-sized granules.

Scientists involved in the $820 million mission continued to mull multiple theories of how the glossy, sandblasted rock formed, including through processes that involved volcanic eruptions, windblown dust or sediments settling out of a body of water.

Spirit still rolling
Halfway around Mars, Opportunity's twin, Spirit, continued to roll toward a crater, traveling nearly 100 feet (30 meters) on Tuesday. NASA planned to send the rover a short distance farther, then pause for a few days for science observations, Cook said.

Spirit is between 280 and 300 feet (90 meters) from the crater nicknamed "Bonneville." Scientists expect the rover will reach its rim and peer into it for the first time in mid-March, Arvidson said.

Scientists consider such impact craters "windows" into the geologic past of Mars, since they can churn up rocks from deep below the surface that may contain evidence of past water activity that the two rovers were designed to seek.